1. Field of the Invention
This invention relates to the methods and apparatus for manufacturing wavy patterned fiber pre-preg that can be used in conjunction with viscoelastic materials to construct composite structures with unique damping properties. Such pre-pregs not only have an aesthetic appeal but also have the distinction of increasing their stiffness as a function of the angle of the fiber, due to the increase in fiber volume fractions as the fiber angle varies in the wavy or sinuous pattern.
2. Description of Related Art
The control of noise and vibration in composite structures is an important area of current research in aerospace, automotive and other industries. For example, spacecraft vibrations initiated by attitude adjusting thrusters, and motors inhibit accurate aiming of antennas and other equipment carried by the craft. Sound induced or structurally borne vibrations can cause severe damage to the craft and its associated equipment during launch.
Additionally, acoustic and vibration energy can be amplified at or near natural structural resonance due to low inherent damping in the materials used to make fairings and other structural components. A practical way of increasing damping and improving acoustic properties in mechanical structures is required.
Composite materials have been used to construct a wide variety of structural elements, including tubes, enclosures, beams, plates and irregular shapes. Objects as diverse as rocket motor housings and sporting goods, notably skis, archery arrows, vaulting poles and tennis rackets have been constructed from composite materials. While composite constructions have offered many significant advantages, such as excellent strength and stiffness properties, and light weight, the poor vibration damping properties have been of concern.
One of the simplest and often very effective passive damping treatments involves the use of thermo-viscoelastic (TVE) materials. These materials, represented by Avery-Dennison""s FT series (FT-1191 is one example), exhibit both elastic and dissipative qualities which make them useful in a number of passive damping treatments.
Some of the first uses of thermo-viscoelastic materials to increase structural damping involved the use of surface patches of aluminum foil and viscoelastic adhesives. Called constrained or embedded-layer damping, these methods produce modest gains in damping.
One of the more common passive damping methods, Constrained Layer Damping or CLD is achieved by bonding a thin layer of metal sheet, usually aluminum, to an existing structure with a viscoelastic adhesive (Kerwin, 1959). Shear strains develop in the viscoelastic material when the original structure bends or extends. Damping occurs when the deformation of the viscoelastic adhesive creates internal friction in the viscoelastic material, generating heat and thus dissipating energy. Compared to an undamped structure, this approach is modestly successful but its effectiveness decreases markedly as the ratio of the thickness of the base structure to the thickness of the viscoelastic material increases (Hwang, et al, 1992).
Co-cured composite-viscoelastic structures are formed when layers of uncured fiber composites (pre-preg) and viscoelastic materials are alternately stacked and cured together in an oven. Damping occurs in these structures when a load causes differential movement of the opposing laminates, causing shearing in the sandwiched viscoelastic material. The various methods that use this concept of differential shearing of the viscoelastic material can be classified by the fiber orientation methods used to induce damping in the TVE material.
Conventional angled ply composite designs use xc2x1xcex8 lay-ups of straight fiber pre-preg materials to encase the viscoelastic layers, and were first proposed by Barrett (1989) in a design for damped composite tubular components. Barrett combined the concepts of constrained layer damping with anisotropic shear coupling in the constraining composite layers to create a tube that achieved both high damping and high axial stiffness. Barrett""s research showed that maximum shearing was experienced at the ends of the tubes and that clamping the constraining layers of the tube at the ends eliminated much of the damping effect, rendering the design impractical for most applications.
Chevron patterned designs also use conventional angled ply (xc2x1xcex8) composite lay-ups of straight fibers but vary the fiber orientation several times throughout the structure in a given laminate. It was first proposed by Benjamin Dolgin (1990) of NASA and implemented by Olcott (1992).
In Olcott""s implementation of Dolgin""s design, each composite layer is comprised of multiple plies of pre-preg composite material arranged in a series of chevron-like patterns. Each composite layer is also comprised of several xe2x80x9csegmentsxe2x80x9d of material where the fiber angle in a given segment is oriented in a single direction throughout its thickness. Segments on opposite sides of the embedded viscoelastic material have the opposite angular orientation. At least two adjacent segments in a given composite layer are required to form a chevron and are joined together by staggering and overlapping the pre-preg plies in the segment.
By tailoring the fiber angle, thickness, and segment lengths, significant shearing in the viscoelastic layer was observed over the entire structure, not just at the ends as in Barrett""s design (Olcott, 1992). Olcott""s research showed that the fiber orientation, segment length, segment overlap length, material choice, and material thickness, had to be carefully controlled to maximize damping in a structure (Olcott, 1992).
Pratt, et. al. (co-pending U.S. application Serial No. 08970141) proposed several processes for making the wavy pre-pregs contemplated by Dolgin, their use in combination with viscoelastic materials for increased damping in composite structures, and the manufacture and use of several specialized wave forms.
The following terms used herein will be understood to have their ordinary dictionary meaning as follows:
The following publications, incorporated herein by reference, are cited for further details on this subject.
1. Dolgin, Benjamin P., xe2x80x9cComposite Passive Damping Struts for Large Precision Structures,xe2x80x9d 1990, U.S. Pat. No. 5,203,435.
2. Dolgin, Benjamin P., xe2x80x9cComposite Struts Would Damp Vibrations,xe2x80x9d NASA Technical Briefs, 1991, Vol. 15, Issue 4, p. 79.
3. Olcott, D. D., xe2x80x9cImproved Damping in Composite Structures Through Stress Coupling, Co-Cured Damping Layers, and Segmented Stiffness Layers,xe2x80x9d Brigham Young University, Ph.D. Thesis, August 1992.
4. Kerwin, xe2x80x9cDamping of Flexural Waves by a Constrained Viscoelastic Layer,xe2x80x9d Journal of the Acoustical Society of America, 1959, Vol. 31, Issue 7, pp. 952-962.
5. Hwang, xe2x80x9cUse of Strain Energy Based Finite Element Techniques in the Analysis of Various Aspects of Damping of Composite Materials and Structures,xe2x80x9d Journal of Composite Materials, 1992, Vol. 26, Issue 17, pp. 2585-2605.
6. Barrett, xe2x80x9cA Design for Improving the Structural Damping Properties of Axial Members,xe2x80x9d Proceedings of Damping, 1989, Vol. HBC-1-18.
7. Horsting, Karl-Heinz, xe2x80x9cMachine for the Production of Pre-Ready Made Reinforcement Formations,xe2x80x9d 1995, U.S. Pat. No. 5,788,804.
8. Ferrentino, Antonio and Beretta, Germano, xe2x80x9cOpitcal Fiber Cable,xe2x80x9d 1976, U.S. Pat. No. 3,937,559.
9. Scharf, Walter G., xe2x80x9cFlecked Metallized Yarn,xe2x80x9d 1968, U.S. Pat. No. 3,361,616.
10. Perrin, Frederic, xe2x80x9cProcess and Device for Applying a Thread onto a Support,xe2x80x9d 1999, U.S. Pat. No. 5,863,368.
11. Ikeda, Kazunari, and Sanada, Koichi, xe2x80x9cMethod of Manufacturing Member for use in Tire Including Engaging Wires Between Gears,xe2x80x9d 1991, U.S. Pat. No. 5,009,732.
12. Darrow, Burgess, xe2x80x9cReinforced Web,xe2x80x9d 1931, U.S. Pat. No. 1,800,179.
Horsting (reference 7 above) in columns 4 and 5 and FIGS. 1-5 presents a design for an apparatus for the deposition of wavy glass fibers in a thermoplastic matrix and/or a pre-preg process. Reference Horsting""s FIG. 1, the process as described relies on the use of a fibrous mat carrier (2) as a substrate for the wavy fiber (11), a pin cushion conveyor (1) for locking the fiber into place during the laying of the fiber, and a roller (4) to push the fiber onto the pins of the pin cushion conveyor (1). A second layer of fibrous mat (3) is added to the top and the combination stripped from the pin cushion conveyor (1) by a stripping device (8) and passed between two heated rollers (6) for final consolidation of matrix and fiber. The keys to Horsting""s process are the pin cushion conveyor (1) which holds the fibers in place, the fibrous mat (2 and 3) which holds the matrix, and the transverse and twisting movement of the roller (4) used to push the fiber onto the mat (2).
The Horsting process suffers from several weaknesses. Firstly, the process relies on the use of a non-standard substrate of fibrous material, presumably to give the substrate enough strength to withstand the insertion and removal of many pins from the pin cushion conveyor (1). Typical pre-preg processes carry a partially cured (and tacky) matrix on a sheet of paper or plastic. Called xe2x80x9cresin paperxe2x80x9d in the industry, typical thickness of matrix on the paper is 0.02 to 0.03 mm, and the paper is similar to common butcher paper in texture, thickness, and strength. Use of such standard resin paper would be impossible on Horsting""s machine since the needles would shred the paper, displace the fibers, and create gaps in the pre-preg.
Secondly, the Horsting process relies on the use of needles to hold the shape of the fibers during the impregnation process. If it is assumed that there is at least a nominal tension on the fibers, the shape of the fibers would conform to a series of straight lines between succeeding pins. Thus, instead of a true sinuous shape, the fiber lay would more closely resemble a piecewise linear approximation of a sinuous wave. Additionally, the fibers would tend to bunch up vertically on the pins as the fibers were bent around the pins. Even if the roller (4) was able to push the fibers onto the matrix in the mat (2), there would still occur areas in the pre-preg where the fiber would be bunched up on the side of the pins. Even after consolidation of the fibers by the heated rollers (6) this bunching of fibers would cause areas in the pre-preg that were alternately fiber-rich or resin-rich. This causes areas of weakness where failures are more likely to occur in cured composite structures made from this pre-preg material. Finally, while bending fibers around small radius pins may be possible for some fibers such as cotton, rayon, etc., for high strength fibers such as carbon or graphite fibers, such small radius bends would result in breakage of the fibers. Experiments by the inventor with bending carbon fiber around pins to create a sinuous waveform resulted in severe problems in the pre-preg with areas of both non-uniform fiber distribution and fiber breakage.
Thirdly, Horsting relies on a roller (4) to push the fiber (11) onto the mat (2) and subsequently onto the needle conveyor (1). The movement of this roller is complex being both transversely to the direction of movement of material through the machine and rotationally about the vertical axis. This type of movement is incapable of creating a uniform sinusoidal waveform across the width. To create a true sinusoidal waveform it is necessary for axis of the roller (4) to maintain a perfectly perpendicular orientation to the direction of travel of material through the machine. If the roller is allowed to rotate about the vertical axis, the waveform on one end of the roller will be materially different than the waveform on the opposite side of the roller. This is because the swept distances of the two ends are different by a function of the apparent axis of rotation about the vertical axis. If the waveforms intended to be produced were sinusoidal (for example), only the fibers in the middle of the roller (4) would create the intended pattern. The patterns at the edges of the pre-preg would more closely resemble the absolute value of the sinusoid i.e., a series of convex semi-circles or scalloped patterns. If the roller is fixed about the vertical rotational axis, then in order to make the sinusoidal waveform, the roller would have to move transversely across the mat (2) with the roller (4) held rotationally fixed. As the roller is moved transversely, the tip of the underlying needles in the conveyor (1) would make a transverse oblique path through the fibers (11) causing some of the fibers to be misplaced in adjacent slots. Thus the waveform of the fibers next to the mat (2) would be of a different pattern than the fibers distant from the mat. Any derivative of such a device that involved the transverse movement of fibers across an array of needles will have the same problem. Additionally, instead of demanding the same amount of fiber across the width of the roller, a greater or lesser amount would be required from the opposite ends of the roller again depending on the apparent axis of rotation of the roller. This would likely cause problems with uniform fiber feed rates, quality, etc. Thus Horsting""s device is incapable of producing a uniform fiber pattern both through the depth and across the width of the pre-preg and is limited in the variety of waveform lengths producible.
Ferrentino et al (reference 8 above) presented a design for an apparatus for embedding optical fibers in a thermoplastic matrix with a sinuous pattern. As described in columns 2-6 and shown in Ferrentino""s FIG. 2, the key to the process is the transverse movement of the comb-like structure (12) which is used to separate the individual optical fiber cables. In many respects, the process shown in FIG. 2 is similar to standard industry practices for making fiber-reinforced pre-preg. If the Ferrentino process were applied to the manufacture of pre-preg where the individual optical fibers were replaced with tows of 15,000 (as an example) fibers, even without the sinuous pattern the resulting structure would resemble a series of fiber ribbons with significant gaps between adjacent tows. This is because the tow has not been flattened out sufficiently to present a uniform distribution of fibers throughout both the thickness and width. A typical pre-preg process requires a series of rollers to flatten the tow and cause even distribution of the fibers prior to encapsulation of the fibers in the resin matrix. This is an absolutely critical step in the production of fiber reinforced pre-preg. Without it, even if there were no visible gaps there would be significant areas of fiber or resin richness that would cause weakness in the pre-preg. Even pre-flattened tow would not be useable in this process since when moved to the side, the tow would ride up on the side of the pins subsequently presenting the tow to be encapsulated in a vertical instead of a horizontal or flattened manner. Thus the Ferrentino method, while useful for the production of embedded optical fibers, is unusable in the production of pre-preg where consistent fiber distribution and consistency in waveform is important.
For the reasons as stated for Ferrentino, Scharf""s method for producing decorative yarn (reference 9) and Perrin""s method for making sinuous pre-preg for tires (reference 10) both suffer from identical weaknesses since both rely on the use of a thread guide and are therefore incapable of producing sinuous pre-preg with uniform qualities across the width and throughout the thickness of the pre-preg. Both Scharf and Perrin contemplated the use of standard yarns for the fibers used to produce the sinuous materials. Perrin contemplated the use of the same methods for the production of graphite, glass or other fiber-reinforced pre-pregs but the reality is that the methods are not transportable for the reasons stated in the discussion of Ferrentino""s apparatus (above). Neither Scharf""s nor Perrin""s methods would be able to produce a pre-preg (even without the sinuous shape) with uniform properties across the width or through the thickness of the pre-preg.
Ikeda et al (reference 11) presented an apparatus for placing sinusoidal bent wires in rubber for the production of tires. Ikeda et al relied on gears to cause a permanent vertical sinuous shape in metallic wires followed by a roller designed to cause them to lay on their side. While such a method may be useful for the permanent set of a metallic wire, such a method cannot be applied to fibers commonly used in the production of fiber-reinforced pre-preg. As an example, graphite fibers are flexible but it is impossible to cause them to have a permanent set. Graphite fiber, glass fiber, rayon, boron, and any other fiber normally used as reinforcement in a pre-preg will not retain a permanent set by the system proposed by Ikeda but will fracture instead. Ikeda et al also claim the use of a comb for spacing the metallic wires. The use of a comb will not permit even distribution of the fibers as is necessary in the production of a fiber reinforced pre-preg as discussed for Ferrentino""s apparatus.
Darrow (reference 12), like Ikeda, proposed a device for obtaining a permanent sinuous waveform in metallic wires for the production of rubber tires. Such methods will not work for fibers commonly used in the production of fiber-reinforced pre-preg as discussed for Ikeda et al.
Dolgin (reference 1) proposed a specialty composite structure made from opposing chevron and sinusoidal patterned composite lamina constraining a viscoelastic layer. In reference 2 Dolgin stated that the production of wavy sinusoidal pre-preg should be possible but did not describe any process or apparatus.
The following paragraphs will discuss a typical pre-preg process as a prelude to understanding the process for producing wavy patterned pre-preg.
In a typical pre-preg process, multiple fiber tows from a creel are moved across a series of rollers, termed xe2x80x9cspreader rollers,xe2x80x9d that progressively spread the fibers until the fibers obtain a uniform thickness and distribution across the width of the rollers. The fibers are then sandwiched between two sheets of coated paper that have a thin layer of partially cured resin. Typical resins are very tacky, have the consistency of taffy, and xe2x80x9cgrabxe2x80x9d the surface fibers when moderate pressure from the combining rollers is applied. This locks the dry fiber, resin coated paper combination together.
This combination of dry fiber and resin paper then moves through a series of heated compaction rollers that thin the resin causing wicking of the resin into the fibers. The spacing between the heated compaction rollers is set to apply enough pressure to further spread the fibers which gives the pre-preg a uniform distribution of fibers, and a uniform thickness. After wicking of the resin into the fiber is completed, the combination is passed over a chill plate that cools the pre-preg, prevents curing, and thickens the fiber-resin combination. The pre-preg is drawn through a set of drive rollers, the top sheet is removed, and the finished pre-preg with its bottom sheet is rolled up on the take-up roller. This example is a simplified explanation that shows key steps in the process. Other steps could include edge trimming, paper backing exchanges, and other special processes. The process thus described is typical of a process used to produce unidirectional pre-preg. With minor modification, cloth pre-preg (made using woven fiber cloth) can be produced using the same basic process.
Typically, all rollers in contact with the fiber-resin paper combination are driven to prevent placing excessive strain on the resin-paper and causing breakage. Heated compaction rollers are typically made of steel with no special coating but the drive rollers are usually coated with a hard rubber coating to improving gripping of the resin-paper combination. Primary drive power is thus provided by the rubber coated drive rollers.
The present invention is directed to an apparatus and method for manufacturing wavy patterned fiber pre-preg materials. Generally, characteristics of the processes and machines include:
Laying the fiber(s) in a controlled wavy pattern that can be periodic or non-periodic; producing a fiber reinforced pre-preg material consisting of matrices containing continuous fibers. The matrices can consist of conventional polymers, viscoelastic materials, resins, or more exotic materials including (but not limited to) metal, ceramic, or combinations of materials. The fibers can consist of unidirectional tow or woven fabric.
The present invention is also directed to a wavy fiber pre-preg material that may be fabricated with a matrix with viscoelastic properties, i.e. the viscoelastic material may comprise all or part of the matrix material binding the fibers) and separate viscoelastic layers as well.
The present invention is also directed to the use of pinch rollers (used to grip the uniformly distributed fibers) attached to a table that moves in both a longitudinal or X and a transverse or Y direction that is used to impart the wavy pattern to the fibers as they are encapsulated in resinous or matrix material. This process permits the production of wavy pre-preg material (especially short wave-length waveforms) with greater consistency, accuracy, and uniformity. Unlike prior methods, this method and apparatus is capable of forming wavy fiber resin-impregnated pre-preg with uniform properties across the width and throughout the thickness of the pre-preg. Unlike prior methods and apparatuses, the present invention is capable of using existing standard materials (both fiber and resin paper) and processes, can be easily retrofitted to existing machines, and will permit the efficient and inexpensive production of both wavy pre-preg as well as conventional pre-preg without modifications or adjustments.